Polypropylene offers a unique combination of mechanical and chemical properties. It has therefore found wide use in a large number of applications. It is generally of great importance to have a well processable material with good mechanical properties such as high flexural modulus and/or impact strength. A good flowability combined with adequate thermal properties are needed for achieving a good processability in various manufacturing methods of articles, thereby allowing a high production speed required for the mass production market.
The mechanical properties are also critical to avoid damage due to storing, handling or dropping of the articles made of polypropylene. Still further, the optical properties should be acceptable. Optical properties are mainly defined by the haze and the gloss of the material. The haze is influenced by the nature and the crystallinity of the polypropylene, lowering the intrinsic crystallinity reduces the haze but also reduces the stiffness. Hence, the balance of stiffness and haze is of great importance.
WO2012/072776 discloses the preparation of caps and closures made of monomodal polypropylene random copolymer (noted PPR) and having improved flexural modulus. The improvement of the flexural modulus is observed when using metallocene catalysts instead of Ziegler-Natta catalysts and simultaneously by lowering the content of the comonomer and increasing the melt flow index of the resulting polypropylene random copolymer. The haze values of the PPR copolymer are also slightly reduced when metallocene catalysts are used in the above-mentioned conditions.
While the manufacturers of polypropylene have continuously worked to improve the properties of polypropylene used in the manufacture of various articles, the ever increasing requirements necessitate the further development of polypropylene.